The present invention relates to a semiconductor laser driving circuit and a semiconductor laser driving method. In particular, the present invention relates to a semiconductor laser driving circuit for driving a plurality of semiconductor laser devices of different objective operating voltages, as in the case of a short-wavelength violet laser and a long-wavelength red laser for performing recording and reproduction on, for example, an optical disc. The present invention relates also to a semiconductor laser driving method for operating such a semiconductor laser driving circuit.
In order to perform recording and reproduction on an optical disc, a semiconductor laser device that has an oscillation wavelength in a 780-nm band is employed in an optical disc apparatus of CD (Compact Disc), MD (Mini Disc) and the like, while a semiconductor laser device that has an oscillation wavelength in a 650-nm band is employed in an optical disc apparatus of DVD (digital versatile disc) and the like.
Conventionally, among optical disc apparatuses to perform recording and reproduction of audio and video data or to be mounted in personal computers, there have been known one in which two kinds of semiconductor laser devices corresponding to two wavelengths are mounted in one housing so as to achieve compatible use for the recording and reproduction of a DVD system and the recording and reproduction of a CD system. The optical disc apparatuses of the above-mentioned kind include an apparatus in which two kinds of semiconductor laser devices are driven by one semiconductor laser driving circuit for the purpose of reductions in apparatus cost and consumption power.
For example, a CD-DVD combination laser driver (article number: EL6290C), which is produced by Elantec Inc., drives two kinds of mounted semiconductor laser devices by means of a driving circuit that uses an identical 5-V power voltage.
Moreover, a 5-channel laser driver (article number: T0800), which can cope with DVD-RAM and CD-RW and is produced by TEMIC Corp. drives two kinds of semiconductor laser devices mounted by means of a current amplification circuit that performs rapid switching with an identical 5-V power voltage.
The oscillation wavelength of the semiconductor laser device is varied depending on the constituent material of the device. Therefore, the semiconductor laser devices constituted of different material have different oscillation wavelengths each other, and the semiconductor laser devices of different oscillation wavelengths have respective inherent objective operating voltages. For example, the objective operating voltage of the semiconductor laser device of the 650-nm band is higher than the objective operating voltage of the semiconductor laser device of the 780-nm band by about 0.5 V (note that each voltage is not higher than about 2 V).
It is herein noted that the power voltage is set giving priority to the semiconductor laser device of the 650-nm band of the high objective operating voltage in the conventional driving circuit that employs an identical power voltage. Therefore, when driving the semiconductor laser device of the 780-nm band of the low objective operating voltage, the power voltage that exceeds the optimum level is used for the driving. This has consequently led to the problem that wasteful power has been consumed by the semiconductor laser driving circuit.
Moreover, in the conventional driving circuit, the power sources of the current amplification circuit corresponding to both the semiconductor laser devices are consistently turned on even when only one semiconductor laser device is driven so that an unnecessary pulse such as short-period surge is not generated at the time of changeover of the semiconductor laser devices. Then, the changeover between light emission and light non-emission is performed by the selection of the semiconductor laser device. Therefore, the current amplification circuit corresponding to the semiconductor laser device of light non-emission has also been operated. This has led a problem that unnecessary electric power has been consumed.
There has been a problem that the temperature inside the optical disc apparatus mounted with the semiconductor laser driving circuit has risen because the greater part of the above-mentioned unnecessary electric power consumed by the semiconductor laser driving circuit has been transformed into heat.
There has also been a problem that the operating time of a battery has been reduced in the case of the battery used as a power source since the semiconductor laser driving circuit consumes the electric power very much.
Moreover, a semiconductor laser device, which has a wavelength in a 400-nm band, has recently been developed. When operating the semiconductor laser device of the 400-nm band, a voltage of about 3 V is needed as a voltage across both ends of the device in order to satisfy the diode characteristics, and a voltage value exceeding 4 V is needed as a voltage across both ends of the device also in the vicinity of the threshold of light emission. As a result, a voltage value (about +9 V) exceeding 5 V is needed as a power voltage for driving the semiconductor laser device of the 400-nm band. If the semiconductor laser device of this 400-nm band is driven with same power voltage (about +9 V) in addition to the aforementioned long-wavelength semiconductor laser device, then the aforementioned problems become more serious, as exemplified by a further increase in wasteful power consumption.